Interpolar spindle microtubules in PTK cells

The formation and functioning of yeast mitotic spindles

The mitotic spindle contains the machinery responsible for sister chromatid segregation. It is composed of a complex and dynamic array of microtubules, which are nucleated from the spindle poles. Studies of yeast spindle functions by molecular genetic analysis and by in vitro functional analysis have identified proteins that are mitosis-specific and present at very low concentrations in the cell, and have revealed the molecular bases of several processes required for the formation and functioning of the mitotic spindle. Here I review the current knowledge of the processes that are common to most eukaryotes: microtubule nucleation at the spindle poles, bipolar spindle assembly, maintenance of the spindle structure, chromosome attachment to the spindle and chromosome separation on the spindle.

Effect on microtubule dynamics of XMAP230, a microtubule-associated protein present in Xenopus laevis eggs and dividing cells

The reorganization from a radial [corrected] interphase microtubule (MT) network into a bipolar spindle at the onset of mitosis involves a dramatic change in MT dynamics. Microtubule-associated proteins (MAPs) and other factors are thought to regulate MT dynamics both in interphase and in mitosis. In this study we report the purification and functional in vitro characterization of a 230-KD MAP from Xenopus egg extract (XMAP230). This protein is present in eggs, oocytes, testis and a Xenopus tissue culture cell line. It is apparently absent from non-dividing cells in which an immunologically related 200-kD protein is found. XMAP230 is composed of two isoforms with slightly different molecular masses and pIs. It is localized to interphase MTs, dissociates from MTs at the onset of prophase and specifically binds to spindle MTs during metaphase and anaphase. The dissociation constant of XMAP230 is 500 nM, the stoichiometry of binding to MTs is between 1:8 and 1:4, and the in vivo concentration is approximately 200 nM. Both isoforms are phosphorylated and have reduced affinity for microtubules in mitotic extracts. Analysis of the effect of XMAP230 on MT dynamics by video microscopy shows that it increases the growth rate, decreases the shrinking rate of MTs and strongly suppresses catastrophes. These results suggest that in vivo, XMAP230 participates in the control of the MT elongation rate, stabilizes MTs and locally modulates MT dynamics during mitosis.

Single particle tracking of surface receptor movement during cell division

We have used fluorescent latex beads to label membrane receptors on cultured NRK cells. Movement of individual beads during cell division was recorded with digital imaging techniques. Surface-bound beads showed no organized movement during metaphase but started to migrate toward the equator approximately 1 min after anaphase onset, when chromosomes moved out of the equatorial region to create the interzone. The movement was most active in the central region of the cell near separating chromosomes, while beads located near the poles of the cell underwent primarily random motion. Most beads showed a surge in speed upon the passage of chromosomes, suggesting a possible link between chromosome separation and cortical reorganization. Furthermore, treatment of anaphase cells with cytochalasin D induced a rapid, simultaneous collapse of beads and cortical actin filaments into aggregates, indicating that the movement of beads was closely related to the reorganization of the actin cortex. In contrast to normal directional movement, cytochalasin-induced movement occurred in random directions and caused some beads in the equatorial region to move toward poles. Our results indicate that cytokinesis involves contractile activities, not only along the equator, but over a wide area of the actin-containing cortex. In addition, organized cortical activities appear to be temporally activated at anaphase onset, and spatially modulated by the spindle interzone or separating chromosomes.

Centrosome and kinetochore movement during mitosis

During the past year important progress has been made in refining our understanding of how chromosomes become equally distributed to daughter cells during mitosis. Unlike the situation in diatoms and yeast, it now appears that spindle pole (centrosome) separation during spindle formation and anaphase B is mediated in vertebrates primarily by an astral pulling, and not a pushing, mechanism. Kinetochore motility is directionally unstable, which has important consequences for how chromosomes move to the equator of the forming spindle. Finally, the observation that sister chromatid disjunction occurs even in the presence of high levels of maturation promoting factor reveals that the series of biochemical events responsible for this phenomenon is not an obligatory part of the pathway by which the cell exits mitosis.